[l2h] PNG transparency again

[Jean-Pierre Chretien]

Hello,

I know this has been discussed already, but I really don't understand
what controls image transparency.

This image has a grey background:

AFPL Ghostscript 8.51 (2005-04-18)
Copyright (C) 2005 artofcode LLC, Benicia, CA.  All rights reserved.
This software comes with NO WARRANTY: see the file PUBLIC for details.
GS>GS>Loading NimbusRomNo9L-Regu font from /usr/local/share/ghostscript/fonts/n021003l.pfb... 2303232 957024 2541896 759840 9 done.
>>showpage, press <return> to continue<<
GS>>>showpage, press <return> to continue<<
GS>Image "/tmp/l2h21346/p22360.pnm" is PPM, 504x491
Running "/usr/local/netpbm/bin/pnmcrop -verbose  < /tmp/l2h21346/p22360.pnm | /usr/local/netpbm/bin/ppmquant -floyd 256 | 
/usr/local/netpbm/bin/pnmtopng -interlace -trans 'gray85'  > my_document-img8.png"
pnmcrop: Background color is gray70
pnmcrop: cropping 35 rows off the top
pnmcrop: cropping 6 rows off the bottom
pnmcrop: cropping 18 cols off the left
pnmcrop: cropping 6 cols off the right
ppmquant: making histogram...
ppmquant: 41 colors found
ppmquant: choosing 256 colors...
ppmquant: mapping image to new colors...
pnmtopng: 41 colors found
pstoimg: Written my_document-img8.png

This one is transparent:

AFPL Ghostscript 8.51 (2005-04-18)
Copyright (C) 2005 artofcode LLC, Benicia, CA.  All rights reserved.
This software comes with NO WARRANTY: see the file PUBLIC for details.
GS>GS>Loading NimbusRomNo9L-Regu font from /usr/local/share/ghostscript/fonts/n021003l.pfb... 2303232 953025 1939016 644789 2 done.
>>showpage, press <return> to continue<<
GS>>>showpage, press <return> to continue<<
GS>Image "/tmp/l2h21346/p22163.pnm" is PPM, 563x92
Running "/usr/local/netpbm/bin/pnmcrop -verbose  < /tmp/l2h21346/p22163.pnm | /usr/local/netpbm/bin/pnmcrop -verbose  -bot  -sides  
| /usr/local/netpbm/bin/pnmcrop -verbose  -l  -sides  | /usr/local/netpbm/bin/pnmcrop -verbose  -r  -sides  | 
/usr/local/netpbm/bin/pnmcrop -verbose  -l  -sides  | /usr/local/netpbm/bin/ppmquant -floyd 256 | /usr/local/netpbm/bin/pnmtopng 
-interlace -trans 'gray85'  > my_document-img14.png"
pnmcrop: Background color is gray70
pnmcrop: cropping 2 rows off the top
pnmcrop: cropping 2 rows off the bottom
pnmcrop: cropping 1 col off the left
pnmcrop: cropping 3 cols off the right
pnmcrop: Background color is black
pnmcrop: Not cropping.  No border found.
pnmcrop: Background color is black
pnmcrop: cropping 2 cols off the left
pnmcrop: Background color is gray70
pnmcrop: Not cropping.  No border found.
pnmcrop: Background color is gray70
pnmcrop: cropping 24 cols off the left
ppmquant: making histogram...
ppmquant: 16 colors found
ppmquant: choosing 256 colors...
ppmquant: mapping image to new colors...
pnmtopng: 16 colors found
pstoimg: Written my_document-img14.png

Netpbm-10.3 here, could this be the problem (current is 10.23).

Moreover, display my_document-ing14.png shows a chessboard of grays, but the 
Firefox display is OK.
(ImageMagick 6.2.6)
 
Any clue since we discussed this ?

TIA

-- 
Jean-Pierre